Chelsea are preparing to make Atlético Madrid strikerDiego Costa their primary transfer target this year, with Wayne Rooney now increasingly likely to sign a new contract at Manchester United.

Rooney has been Jose Mourinho’s priority signing since he returned to Chelsea last summer but there is a growing sense that it will be impossible to prise him from United and other long-term options must now be pursued.

Costa has been on Chelsea’s radar for several months but it is understood that a fresh inquiry was made this week and, although any deal would almost certainly have to wait until the summer, he is regarded as an attainable target.

Chelsea have loaned goalkeeper Thibaut Courtois to Atlético Madrid during the past three seasons and could also be aided in any deal by the fact that Jorge Mendes is the agent to both Costa and Mourinho.

It was suggested in Spain on Monday that Atlético would ask to include Courtois in a permanent deal for Costa. Chelsea, though, regard Courtois as potentially the best goalkeeper in the world and would be extremely reluctant to lose him permanently.

Still only 21, Courtois has benefited hugely from regular first-team football in Spain and is seen as the ideal long-term replacement to Petr Cech.

Costa has a buyout clause of £32 million. Arsenal are also interested in Costa, whose goals and outstanding performances have helped Atlético sustain an unlikely three-way challenge with Barcelona and Real Madrid for La Liga.

One player to leave Stamford Bridge on Monday was Michael Essien, with Mourinho deciding to accept the midfielder’s requested move to AC Milan out of “respect” for his stellar Chelsea career.

Essien had won every major club honour during more than eight years with Chelsea but has become peripheral this season and his opportunities will be further limited by the return of Nemanja Matic.

Chelsea have also bought Mohamed Salah this month from Basle, a player likened yesterday to the club’s former winger Arjen Robben.

“He is a great player, we know him because we played him in the last two years, four or five times,” said Branislav Ivanovic.

“With the ball he is very quick, very smart with his movement, the kind of player that we need in the squad. He reminds me of Robben, the way he plays always going from the right wing but going inside left-footed.”

Ivanovic also believes that Matic is much improved following his three years with Benfica.

“When I saw him when he came back he was bigger and stronger,” said Ivanovic.